In subterranean well operations, it is necessary from time to time to set a whipstock in a subsurface well conduit such as a tubing string or a well casing. The whipstock is set to deviate a mill bit or a drill bit away from the longitudinal axis of the conduit to mill a window in the conduit from which to drill a deviated well bore at an angle to the longitudinal axis of the conduit.
The cost and time consumed in using a conventional rotary drilling rig in the foregoing situation is considerable and there has been a trend towards the use of coiled tubing units for these and other well operations heretofore conducted with conventional (jointed straight pipe) drilling rigs.
Coiled tubing units are known in the art, but not widely used in the field yet. Coiled tubing units are nevertheless available on a commercial basis. Inventions such as that disclosed herein will render coiled tubing units more readily useful in the field by reducing both the cost and time expenditure, as compared to a conventional drilling rig, for a given operation.
Heretofore, tools and procedures have been developed for use with conventional drilling rigs for various operations such as removing (milling) a section of a well conduit, whether it is tubing or casing, but these tools and procedures cannot be transferred unchanged to a coiled tubing unit and employed successfully in the same manner as employed in the conventional drilling rig. The use of conventional drilling rig tools and procedures in a coiled tubing context has several shortcomings. For example, control over the axial downward pressure on the tool or tools employed downhole is difficult to maintain because of the flexibility of the coiled tubing string. Accordingly, the cutting or milling tool may wear prematurely or unduly cut into other downhole tools such as whipstocks.